The present invention relates to integrated circuit mounting techniques, and more specifically to an apparatus and method for stacking integrated circuit devices.
As computers get smaller, printed circuit board area also tends to get smaller. At the same time, computers are becoming more powerful, causing problems with fitting circuitry into a limited printed circuit board area. For example, a major problem exists in mounting cache and RAM memory onto printed circuit boards. Newer, more powerful computers require larger amounts of cache and RAM memory for server operation, multi-tasking operation, sound processing, moving video processing, and graphics processing.
One proposal for solving this problem is to stack multiple laminate layers, such as those manufactured by Staktek, Inc., and RTB Technologies, Inc., one on top of the other. Each laminate layer provides a frame within which a SRAM or DRAM chip is mounted, as well as connections to the chip and other laminate layers. The RAM chips are connected to the laminate frames by a wire bonding process known in the art.
However, the wire bonding method suffers from the disadvantage that the bond wire is a major contributor to signal parasitic inductance, especially if the wire loop length is not strictly minimized. As processor and cache clock frequencies increase, parasitic inductance distorts signals between the processor and a planar array of multiple cache chips.
Therefore, it would be desirable to provide a method for stacking integrated circuit chips, such as cache and RAM chips, which minimizes signal parasitic inductance. The present invention proposes that modern flip-chip technology be used instead of wire bonding methods. An excellent discussion of flip-chip process steps may be found in the book "Multichip Module Technologies And Alternatives: The Basics", edited by Daryl Ann Doane and Paul D. Franzon, and published by Van Nostrand-Reinhold, New York, in 1993. This book is hereby incorporated by reference.
The basic connection scheme consists of die input/output (I/0) pads that have had solder bumps applied, plus a matching set of substrate solder wettable pads. The die I/O pads are formed by etching vias through the passivation layer followed by hermetically sealing the via by evaporating layers of chromium, copper, or gold through an appropriate mask. A solder alloy is then deposited on the pad to form the solder bump. The melting point of the solder is high enough to ensure compatibility with subsequent assembly steps. Meanwhile, the substrate solder wettable pads are formed so molten solder is restrained from flowing outward from the bump causing a controlled collapse of the bump during the subsequent solder reflow step. The reflow step can be achieved in a vapor-phase or infrared oven or by a localized heat source.